The calcium-dependent calmodulin interaction of microtubule-associated protein 2 MAP2 has been studied by cross-linking of chymotryptic fragments of MAP2. Several approaches show that the calmodulin attachment site is on the short arm of MAP2 that binds to tubulin and not on the long side arm that interacts with cellular organelles. We have also reinvestigated the basis for the fluorescence that occurs on binding of colchicine to tubulin. This is not primarily a function of the hydrophobic environment provided by the binding site but is a function of the immobilization of the drug in the site and can be mimicked by high viscosity, protein-free, media or by non-specific covalent coupling to proteins that do not normally bind colchicine.